1. Field of the Invention
The present invention relates to the field of telecommunications. More particularly, the present invention relates to improving performance in switch based telecommunications networks employing virtual connections, such as switched virtual connections (SVCs). The telecommunications network may include virtual tandem switches employing asynchronous transfer mode (ATM) networks.
2. Background Information
In standard call processing, cross-office delay must be below an acceptable level in order to minimize the duration of silence after a telephone call has been dialed. The signaling channel message processing required for standard call processing is well-studied and well-specified for conventional time division multiplexed (TDM) circuit-switched voice networks. ITU-T, xe2x80x9cSpecifications of Signaling System No. 7 ISDN User Partxe2x80x9d, ITU-T Recommendation Q.766, March, 1993; and Bellcore, xe2x80x9cLSSGR: Switch Processing Time Generic Requirements, Section 5.6xe2x80x9d, GR-1364-CORE, Issue 1, June, 1995, are specifications discussing such processing. These specifications dictate the cross-office delay requirements for processing of a Signaling System No.7(SS7) messages.
With reference to FIG. 1 of the drawings, standard call processing employs end offices 10 connected via tandem trunks 12, direct trunks 14, or both tandem 12 and direct trunks 14. Each trunk 12, 14 is a digital service level 0 (DS0), operating at 64 kbps, that is transmitted between the switching offices 10 in a time division multiplexed manner. Each end office 10 connects to its neighboring end office 10 and the tandem office 16 using separate trunk groups. In this system, trunk groups are forecasted and pre-provisioned with dedicated bandwidth, which may lead to inefficiency and high operations cost.
A new voice trunking system using asynchronous transfer mode (ATM) technology has been proposed in U.S. patent application Ser. No. 09/287,092, entitled xe2x80x9cATM-Based Distributed Virtual Tandem Switching System,xe2x80x9d filed on Apr. 7, 1999, the disclosure of which is expressly incorporated herein by reference in its entirety. In this system, shown in FIG. 2, voice trunks from end office switches 20, 26 are converted to ATM cells by a trunk inter-working function (T-IWF) device 22, 24. The T-IWFs 22, 24 are distributed to each end office 20, 26, and are controlled by a centralized control and signaling inter-working function (CS-IWF) device 28. The CS-IWF 28 performs call control functions as well as conversion between the narrowband Signaling System No. 7 (SS7) protocol and a broadband signaling protocol. The T-IWFs 22, 24, CS-IWF 28, and the ATM network 30 form the ATM-based distributed virtual tandem switching system. According to this voice trunking over ATM (VTOA) architecture, trunks are no longer statistically provisioned DS0 time slots. Instead, the trunks are realized through dynamically established switched virtual connection (SVCs), thus eliminating the need to provision separate trunk groups to different destinations, as done in TDM-based trunking networks.
The actions necessary in each office are clearly defined upon reception of a particular SS7 message when operating within the standard network. For a normal tandem trunk call flow, the originating end office sends an Initial Address Message (IAM) to the tandem switch through an SS7 network. The IAM message includes a routing address of the tandem office, calling telephone number, called telephone number, and Trunk ID. The tandem switch has a mean processing delay budget of 180 ms as specified in xe2x80x9cSpecifications of Signaling System No. 7 ISDN User partxe2x80x9d (360 ms for 95th percentile) to process the IAM message and to reserve a trunk in the trunk group that is pre-established to the terminating end office.
In voice trunking over ATM (VTOA) technology, a standard time division multiplexed (TDM) tandem is replaced by three components: a trunk inter-working function (T-IWF), a control and signaling inter-working function (CS-IWF), and an ATM network. The three component architecture (i.e., T-IWFs, CS-IWF, and ATM network) requires signaling channel message processing different from IDM processing but must maintain at least the performance of standard TDM-based network processing. That is, these three components should share the 180 ms (mean) budget, as they are considered to be a unique entity, i.e., a virtual tandem switching system. Hence, the time for the ATM network to establish a switched virtual connection (SVC), which is VTOA""s equivalent to reserving a trunk, is stringent.
In VTOA architecture, the end offices and the virtual tandem (i.e., CS-IWF) communicate through an SS7 network, as seen in FIG. 2, the same way the switching offices communicate in TDM-based trunking networks. However, control/signaling and through-connect establishment (an SVC through the ATM network) functions reside in the CS-IWF, and the ATM network and T-IWF, respectively. Coordinating the different components adds new message exchanges into the processing.
In the VTOA architecture,the CS-IWFs have two options upon receiving an IAM message. The first option is to send a message to either an originating or terminating T-IWF for initiation of an ATM connection and wait for an xe2x80x9cATM SVC Establishedxe2x80x9d message before sending the IAM message to the terminating end office. The second option is to send the IAM message to the terminating end office at the same time it sends a request to either T-IWF for an ATM connection establishment. It is expected that the ATM connection will be ready before the reception of Address Complete Message (ACM), which indicates that ringing is applied to the callee and the through-connect should be established in the tandem. The second option provides more time for the establishment of an SVC through ATM network. However, an SVC may very well go through several ATM switches, which generally have reasonably large figures for call setup latency. Although some exceptions exist, it would be unreasonable to assume the latency is low because the latency numbers of new switches are yet to be tested, and already deployed ATM switches can be assumed to serve years to come. In other words, for either option there exists a need for fast SVC setup through the ATM network to stay within the standardized delay budget limits.
One solution to the latency problem is to construct an overlay PVP (Permanent Virtual Path) network in the ATM backbone. With a PVP network, only end points of virtual paths require call processing and transit nodes are not involved in the establishment of SVCs. Further, the design of virtual path networks has been well studied and thus many proposed optimization algorithms exist. However, the efficient management of virtual path networks is still a challenging task in practice. Although constructing an elastic virtual path, which resizes itself with the changing traffic conditions, is a promising solution, there is currently no standard procedure for automatically changing the capacity of virtual paths. Consequently, a telecommunications carrier would have to commit to a proprietary solution, which has its own disadvantages. Finally, PVP networks suffer from the drawback of requiring manual rerouting in case of a network failure. In contrast SVCs are rerouted automatically by the Private Networkxe2x80x94Network Interface (PNNI) routing protocol without interference from the management system in case of failures in ATM network. For management and operations purposes, this feature makes the SVCs highly appealing.
In view of the foregoing, the present invention is directed to improving the performance of VTOA systems. The present invention reduces the total number of SVCs in the ATM network, improves bandwidth utilization, and eliminates a need for manual cache management.
According to an aspect of the present invention an adaptive SVC caching system and method overcome the limitations of ATM switches discussed above by delaying release of SVCs. That is, an already established SVC is not immediately released when a conversation finishes (i.e., when either side hangs up). Instead, the SVC is kept alive for a variable duration, referred to as a caching time, with the expectation that during that time another call request for the same terminating end office will arrive. The caching duration is adaptively changed based upon a call arrival rate and call setup delay experienced in the ATM network in order to stay within the required delay budget. Thus, the processing load of the ATM network is constantly monitored and the caching time is changed accordingly. Preferably, the caching time is increased when the call setup time exceeded the budget, and is decreased when the call setup time was less than required. The present invention successfully tracks changes in the processing load of the ATM network (call setup delay) and in the call arrival rate.
According to an aspect of the present invention, an adaptive switched virtual circuit (SVC) caching method is provided for use within a telecommunications network. The method includes defining a delay budget; estimating a call arrival rate in the network; and estimating a call setup delay in the network. The method also includes determining a cache duration based upon the delay budget, the estimated call arrival rate, and the estimated call setup delay. When an SVC is cached for the cache duration, the caching facilitates processing telephone calls in the network within the delay budget by eliminating call processing for new SVC establishment when a new call request to the destination occurs during the cache.
According to a preferred embodiment, the cache duration is inversely related to the call setup delay. More preferably, the cache duration tcache is calculated from the equation:             t      cache        ⁢          (      n      )        ≈            1              β        ⁢                  ⟨          λ          ⟩                ⁢                  (                      n            -            1                    )                      ⁢          log      ⁢              (                                                            ⟨                d                ⟩                            setup                        ⁢                          (                              n                -                1                            )                                            d            budget                          )            
where:
 less than xcex greater than  is an estimate of the mean call arrival rate.
 less than d greater than setup is an estimate of the mean call setup delay in an ATM network;
dbudget is the delay budget;
xcex2 is a predetermined constant between zero and one; and
n is the time when the call arrival rate and the call setup delay are measured.
According to a preferred embodiment, estimating the call arrival rate includes periodically measuring the call arrival rate at a predetermined interval. Estimating the call setup delay in the network includes periodically measuring the call setup delay in the network at a predetermined interval.
According to an aspect of the present invention, an adaptive switched virtual circuit (SVC) caching method is provided for use within a telecommunications in the network; and estimating a call setup delay in the network. The method also includes determining a cache duration based upon the delay budget, the estimated call arrival rate, and the estimated call setup delay. The method further includes establishing an SVC to a destination in response to a telephone call to the destination; caching the SVC for the cache duration after the telephone call terminates; reusing the cached SVC when a new call request to the destination occurs during the cache; and releasing the cached SVC after the cache duration when no new call request to the destination occurs during the cache. The cached SVC facilitates processing telephone calls in the network within the delay budget by eliminating call processing for new SVC establishment when the new call request to the destination occurs during the cache.
According to a preferred embodiment, estimating the call arrival rate includes periodically measuring the call arrival rate at a predetermined interval. Estimating the call setup delay in the network includes periodically measuring the call setup delay in the network at a predetermined interval. Measuring the call setup delay may include measuring the time between transmitting an initial setup message from an originating T-IWF and receiving a final connect message at the originating T-IWF.
According to a preferred embodiment, the cache duration is inversely related to the call setup delay. More preferably, the cache duration tcache is calculated from the equation:             t      cache        ⁢          (      n      )        ≈            1              β        ⁢                  ⟨          λ          ⟩                ⁢                  (                      n            -            1                    )                      ⁢          log      ⁢              (                                                            ⟨                d                ⟩                            setup                        ⁢                          (                              n                -                1                            )                                            d            budget                          )            
where:
 less than xcex greater than  is an estimate of the mean call arrival rate.
 less than d greater than setup is an estimate of the mean call setup delay in an ATM network;
dbudget is the delay budget;
xcex2 is a predetermined constant between zero and one; and
n is the time when the call arrival rate and the call setup delay are measured.
According to a preferred embodiment, the estimate of the mean call arrival rate is filtered, and the estimate of the mean call setup delay in the ATM network is filtered. Preferably, the estimate of the mean call arrival rate is filtered according to the equation:
xe2x80x83 less than xcex greater than (i)=(1xe2x88x92w) less than xcex greater than (ixe2x88x921)+w less than xcex greater than (i)
and the estimate of the mean call setup delay in the ATM network is filtered according to the equation:
 less than d greater than setup(i)=(1xe2x88x92w) less than d greater than setup(ixe2x88x921)+W less than d greater than setup(i)
where w is a weight, and i is a unit of time. Preferably w=0.1. Moreover, a longest cached SVC is selected for use when more than one cached SVC is available for the destination.
According to another aspect of the present invention, a telecommunications system is provided for adaptive switched virtual circuit (SVC) caching. The telecommunications system has a predefined delay budget. The system includes an ATM network having a call arrival rate and a call setup delay; and at least one SVC within the network, the SVC being established to a destination in response to a telephone call to the destination. The system also includes a plurality of T-IWFs that estimate the call arrival rate and the call setup delay. Each T-IWF determines a cache duration based upon the predefined delay budget, the estimated call arrival rate, and the estimated call setup delay. The system also includes a CS-IWF. The SVC is cached for the cache duration after the telephone call terminates. In addition, the cache, and the cached SVC is released after the cache duration when no new call request to the destination occurs during the cache. The cached SVC facilitates processing telephone calls in the ATM network within the delay budget by eliminating call processing for new SVC establishment when the new call request to the destination occurs during the cache.
According to a preferred embodiment, the cache duration is inversely related to the call setup delay. More preferably, each T-IWF calculates the cache duration tcache from the equation:             t      cache        ⁢          (      n      )        ≈            1              β        ⁢                  ⟨          λ          ⟩                ⁢                  (                      n            -            1                    )                      ⁢          log      ⁢              (                                                            ⟨                d                ⟩                            setup                        ⁢                          (                              n                -                1                            )                                            d            budget                          )            
where:
 less than xcex greater than  is an estimate of the mean call arrival rate;
 less than d greater than setup is an estimate of the mean call setup delay in the ATM network;
dbudget is the delay budget;
xcex2 is a predetermined constant between zero and one; and
n is the time when the call arrival rate and the call setup delay are measured.
According to a preferred embodiment, the estimate of the mean call arrival rate is filtered, and the estimate of the mean call setup delay in the ATM network is filtered. Preferably, the estimate of the mean call arrival rate is filtered by the equation:
 less than xcex greater than (i)=(1xe2x88x92w) less than xcex greater than (ixe2x88x921)+w less than xcex greater than (i)
and the estimate of the mean call setup delay in the ATM network is filtered by the equation:
 less than d greater than setup(i)=(1xe2x88x92w) less than d greater than setup(ixe2x88x921)+w less than d greater than setup(i)
where w is a weight, and i is a unit of time.
According to a preferred embodiment, the T-IWFs estimate the call arrival rate by periodically measuring the call arrival rate at a predetermined interval. Further, the T-IWFs estimate the call setup delay in the network by periodically measuring the call setup delay in the network at a predetermined interval. An originating T-IWF measures the call setup delay by measuring the time between transmitting an initial setup message from the originating T-IWF and receiving a final connect message at the originating T-IWF. Preferably, the T-IWF selects a longest cached SVC for reuse when more than one cached SVC is available for the destination.